i like doing some gesture drawings and other fast figure drawings, using sites like line-of-action to create timed images. this helps me get my hand up to speed and get some practice on accuracy, gesture, and seeing and replicating.
similarly, sometimes ill warm up with a study of some objects or designs or costumes relevant to the project at hand, or play around with some new brushes / materials.
i like to do my warmups first in a different medium to how im working - i do lots of pencil and pen and ink warmups and sketches to get me ready to draw digitally.